The present invention relates to a heat-developable photosensitive image-recording element and, more particularly, to an image-recording element which has an alkali-generating system incorporated therein.
In conventional photographic systems, a light-sensitive photographic element containing a photosensitive silver halide emulsion layer is exposed to form a latent image, then the exposed silver halide is developed to a visible silver image by a developer solution. Such a developer solution typically contains an alkaline activator to obtain a pH at which the silver halide can be effectively developed. It is well known in the art that, in general, the developer activity increases as the mount of alkali in the developer is increased, that is, as the pH of the solution is increased above 7.5. A developer solution containing no alkali, produces very little if any silver image development in an exposed photographic element.
The alkaline environment required for development may be provided by any of a number of known techniques. For example, an aqueous alkaline processing composition may be distributed to the image-recording element after exposure of the photosensitive silver halide emulsion such as from a rupturable container as is well known in the diffusion transfer photographic art. Alternatively, the alkali may be generated in situ in the manner described in U.S. Pat. Nos. 3,260,598; 4,740,363; and 4,740,445.
The term "alkali" is generally defined as a material that is generated for the purpose of, or which causes, substantial deprotonation of a component of the thermally processable system such as organic and inorganic salts of hydroxide, e.g., alkali metal hydroxides, ammonium hydroxides, and highly basic organic materials.
It is well known in the art that providing the alkaline environment required for development by incorporating alkaline compounds in photosensitive materials which are then stored for a period of time may result in the discoloration of color images formed and the coloration of white areas due to, among other things, the action of coexisting silver halide, silver complex, developing agents, the hydrolysis of gelatin, and the like. Additional disadvantages exist with respect to using alkalies, i.e., irritation of the skin upon direct contact with the alkali, neutralization of the alkalinity due to absorption of carbon dioxide in air, susceptibility to aerial oxidation and, in general, the lessened stability of developer solutions having increased pH.
As mentioned previously, the alkaline environment required for development may be generated in situ by incorporating an alkali-generating system in the image-recording element as opposed to, for example, being distributed to the image-recording element as an aqueous alkaline processing composition from a rupturable container.
U.S. Pat. No. 3,260,598 discloses a system in which the alkali needed for developing agent activation is generated in the area where development is to take place. The system uses a light-sensitive photographic element that incorporates in the light-sensitive layer or an adjacent layer an alkali-releasing agent comprising a slightly water soluble metal hydroxide, the element being developed with a low alkali developer solution containing an alkali-releasing reagent which reacts with the metal hydroxide to form a substantially less dissociated compound and release the hydroxyl ions needed to activate the development reaction.
U.S. Pat. No. 4,740,363 discloses a process for generating alkali which includes reacting a complexing agent with a slightly water-soluble metal compound in the presence of water.
U.S. Pat. No. 4,740,445 discloses an image forming reaction system including a difficultly soluble metal compound and a compound capable of water-mediated complexing reaction with the metal ion of the difficultly soluble metal compound and having an organic base, by reacting the two compounds in the presence of water to increase the pH of the system.
Heat-developable photosensitive imaging materials are well known in the art, including thermally developable black and white, as well as, color photosensitive materials. Image-recording materials useful in photographic imaging systems including any of the known diffusion transfer color photographic processes such as thermographic and photothermographic typically contain a support carrying a photosensitive silver halide emulsion and a silver halide developing agent.
A visible image is formed in photothermographic image-recording elements by exposing the photosensitive silver halide emulsion to an imagewise pattern of activating light to form a latent image and subsequently applying heat to the element in the presence of the developing agent. The photosensitive silver halide emulsion may serve as the sole source of silver for forming the final image, e.g., the light-sensitive silver may be developed to form a final negative image in reduced (metallic) silver. Alternatively, the photosensitive silver halide emulsion may not be the sole source of silver for forming the final image, e.g., a non light-sensitive source of silver such as a silver salt may be utilized. For example, the exposed photosensitive silver halide emulsion, upon being heated, catalyzes an oxidation-reduction reaction between the non light-sensitive silver salt and the developing agent to form a visible image.
As the state of the art for image-recording elements of this type advances, novel techniques and materials continue to be developed by those skilled in the art in order to attain the performance criteria required of such materials. The present invention relates to a novel heat-developable photosensitive image-recording element and, more particularly, to an image-recording element which has the novel alkali-generating system incorporated therein.